The capacity of the olfactory epithelium (OE) for replenishing the population of olfactory sensory neurons and for regenerating the epithelium after injury depends on the persistence and maintained function of stem cells within that adult epithelium. Decline in sensory function in the elderly is accompanied by pathological changes in the OE that emerge because the normally active olfactory stem and progenitor cells, namely globose basal cells (GBCs), become disordered and eventually depleted. At present, we can construct a flow diagram designed to encompass and sequence the various categories of GBCs, beginning with GBCs that function as multipotent progenitors, progressing through GBCs that act as transit amplifying progenitors, and reaching GBCs that give rise directly to neurons. We can also align those purported stages in the GBC hierarchy with the expression of various transcription factors. However, we lack a comprehensive understanding of this critical stem/progenitor cell population. Are these stages discrete or are they snapshots of a more fluid progression? How do we explain the plasticity in the progenitor capacity of some GBCs? How might aging disorder the progression? The current application proposes two specific aims designed to address the critical gaps in our knowledge of the active stem and progenitor cell population. Specific Aim 1 will generate transcriptomic profiles of the various functional categories of GBCs using transgenic mouse strains that express a fluorescent marker in conjunction with the transcription factor(s) gene(s) ? Sox2, Ascl1, and Neurog1 ? used to define the stages in the hierarchy. Specific Aim 2 will focus on and profile a specific kinetically- defined subset of GBCs that experience prolonged mitotic quiescence, as shown by the retention of the tagged histone fusion protein H2B/GFP, which is a feature common to many stem cell types. In both cases, cells labeled by expression of the fluorescent tags will be isolated by FACS, captured as single cells, and individually profiled. Analysis of the single cell expression libraries will be used for unbiased clustering of the cells and defining the genes whose expression differs across the clusters. The patterns of gene expression that differentiate the clusters will be validated by Q-PCR and either immunohistochemistry or in situ hybridization depending on antibody availability. At the conclusion of the analysis we will have achieved a comprehensive understanding of the GBC population and will clarify whether the existing flow diagram ? which envisions distinct stages and abrupt transitions ? is an accurate representation of the biology of this stem and progenitor cell-encompassing population. These advances, in turn, will inform our attempts to alleviate olfactory sensory dysfunction, particularly that which accompanies aging.